Spin Singlet Formation from S = 1/2 Tetrahedral Clusters

Abstract

Muon spin relaxation ($\hat{I}¼$SR) and nuclear magnetic resonance (NMR) experiments revealed that the spin singlet state with an excitation gap of $\sim$200 K is realized from $S=1/2$ Nb${}_4$ tetrahedral clusters in a cluster Mott insulator GaNb${}_4$S${}_8$. The intercluster cooperative phenomenon to the singlet state at $T_{\mathrm{S}}=32$ K is triggered by intracluster Jahn-Teller type structural instability developed from $\sim$$3T_{\mathrm{S}}$. Referring to the lattice symmetry, the formation of Nb${}_8$ octamer (Nb${}_4$--Nb${}_4$ bond) is suggested.

11 pages, 5figures